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What is FPC

What is FPC
A flexible Printed Circuit (FPC for short) is a highly reliable and excellent flexible printed circuit board made of polyimide or polyester film.
It has the characteristics of high wiring density, lightweight, thin thickness, and good bendability.
 
PCBs are used in electronic products, and the market for PCBs is almost a weather vane for the electronics industry.
With the development of high-end, miniaturized electronic products such as mobile phones, laptops, and PDAs, the demand for flexible PCBs (FPCs) is growing and PCB manufacturers are accelerating the development of thinner, lighter, and higher-density FPCs.

FPC component materials

  •  Insulation material
The insulating film forms the base layer of the circuit and the adhesive bonds the copper foil to the insulating layer. In multilayer designs, it is then bonded to the inner layer. They are also used as protective coverings to insulate the circuit from dust and moisture and to reduce stress during flexing, and the copper foil forms the conductive layer.
In some flexible circuits, rigid members formed from aluminum or stainless steel are used, which provide dimensional stability, physical support for the placement of components and wires, and stress relief. An adhesive bonds the rigid members to the flexible circuit. Another material that is sometimes used in flexible circuits is adhesive laminate, which is formed by coating both sides of an insulating film with an adhesive. Bonding laminates provide environmental protection and electrical insulation and can eliminate one layer of film as well as having the ability to bond multiple layers with fewer layers.
There are many types of insulating film materials, but the most commonly used are polyimide and polyester materials. Nearly 80% of all flexible circuit manufacturers in the US use polyimide materials, with another 20% using polyester film materials. Polyimide materials are non-flammable, geometrically stable, have high tear strength, and have the ability to withstand soldering temperatures, while polyester, also known as polyethylene terephthalate, has similar physical properties to polyimide, has a low dielectric constant and absorbs little moisture, but is not resistant to high temperatures. It is not resistant to high temperatures. The melting point of polyesters is 250°C and the glass transition temperature (Tg) is 80°C, which limits their use in applications requiring extensive end welding. In low-temperature applications, they exhibit rigidity. Nevertheless, they are suitable for use in products such as telephones and other products that do not require exposure to harsh environments for use. Polyimide insulating films are usually combined with polyimide or acrylic bonding agents, polyester insulating materials are usually combined with polyester bonding agents. The advantage of combining materials with the same properties is that they can be dimensionally stable after dry soldering or after several lamination cycles. Other important properties in bonding agents are a low dielectric constant, a high insulation resistance, a high glass transition temperature and a low moisture absorption rate.
 
  • Conductors
Copper foil is suitable for use in flexible circuits and can be either electrodeposited (ED) or plated. One side of the electrodeposited copper foil has a glossy surface, while the other side is processed with a dull, matt surface. It is a flexible material and can be made in many thicknesses and widths. The non-glossy side of the ED copper foil is often specially treated to improve its bonding ability. In addition to its flexibility, wrought copper foil is also hard and smooth, making it suitable for applications where dynamic flexing is required.
 
  • Bonding agent
In addition to being used for bonding insulating films to conductive materials, bonding agents can also be used as a cover layer, as a protective coating, and as an overlay coating. The main difference between the two is the application method used, with overlay bonding covering insulating films to form laminated circuits. The screen printing technique used for the adhesive overlay coating. Not all laminated constructions contain an adhesive; laminations without an adhesive form a thinner circuit and are more flexible. It has a better thermal conductivity than laminated constructions with adhesive-based laminates. Due to the thin construction characteristics of the adhesive-free flexible circuit and the improved thermal conductivity due to the elimination of the thermal resistance of the adhesive, it can be used in operating environments where adhesive-based laminated structures of flexible circuits cannot be used.
 

FPC construction

  • Copper Film
  • Copper foil: There are basically two types of copper: electrolytic copper and calendered copper. Commonly available in thicknesses of 1oz 1/2oz and 1/3 oz.
  • Substrate film: Commonly available in 1 mil and 1/2 mil thicknesses.
  • Adhesives: thicknesses are determined by customer requirements.
  • Cover Film
  • Cover Film: for surface insulation. Commonly available in 1 mil and 1/2 mil thicknesses.
  • Adhesive: thickness to be determined by the customer.
  • Release paper: to avoid adhesives from adhering to foreign objects before lamination; to facilitate work.
  • Reinforcing sheet (PI Stiffener Film)
  • Stiffener Film: reinforces the mechanical strength of the FPC to facilitate surface mounting. Commonly available in thicknesses from 3mil to 9mil.
  • Adhesive: thickness to be determined by customer requirements.
  • Release paper: to avoid adhesives from adhering to foreign objects prior to pressing.
  • EMI: electromagnetic shielding film to protect the circuitry inside the board from external interference (strong electromagnetic zones or areas susceptible to interference).
FPC
FPC
FPC
FPC

FPC classification

  • Single layer FPC
With a layer of chemically etched conductive graphics, the conductive graphics layer on the surface of the flexible insulating substrate is calendered copper foil.
The insulating substrate can be polyimide, polyethylene terephthalate, acrylamide fiber ester, and polyvinyl chloride.
Single-layer FPC can be subdivided into four sub-categories as follows.
  1. Single-sided connection without cover layer. The interconnection is achieved by soldering, fusion welding, or pressure welding and is commonly used in early telephones.
  2. Single-sided connection with a cover layer. Compared with the previous category, there is only an additional layer of cover on the surface of the wire. The pad needs to be exposed when covering, simple ones can be left uncovered at the end area. Single-sided flexible PCB is the most used, the most widely used in automotive instruments, and electronic equipment.
  3. Double-sided connection without cover layer. Connection plate interface in the front and back of the wire can be connected, the insulating substrate at the pad to open a channel hole, the channel hole can be made in the desired location of the insulating substrate first punching, etching, or other mechanical methods.
  4. Double-sided connection with a cover layer. The previous category differs in that there is a cover layer on the surface, which has an access hole, allowing it to be terminated on both sides and still maintain the cover layer, made of two layers of insulating material and a layer of metal conductors.
  • Two-sided FPC
The double-sided FPC has an etched conductive pattern on each side of the insulating base film, increasing the density of wiring per unit area. Metallised holes connect the graphics on both sides of the insulating material to form a conductive pathway to meet the design and use function of flexibilities. The cover film protects the single and double-sided wires and indicates the position of the components. Metallised holes and overlays can be used with or without the FPC, depending on requirements, and there are fewer applications for this type of FPC.
  • Multi-layer FPC
Multi-layer FPC is a lamination of three or more layers of single or double-sided flexible circuits, which are drilled and plated to form metallized holes and conductive paths between the different layers. This eliminates the need for complex soldering processes. Multilayer circuits offer a huge functional difference in terms of higher reliability, better thermal conductivity, and easier assembly properties.
The advantages are the light weight of the substrate film and excellent electrical properties such as low dielectric constants. Polyimide film is a substrate made of multilayer flexible PCB board, then the rigid epoxy glass cloth multilayer PCB board weight is about 1/3 lighter, but it loses the single-sided, double-sided flexible PCB excellent flexibility, most of these products are not required to be flexible. Multilayer FPC can be further divided into the following types.
  1. Flexible insulating substrate finished products. This category is manufactured on a flexible insulating substrate, the finished product is specified to be flexible. The construction is usually a number of singles- or double-sided microstrip flexible PCBs with both ends bonded together, but with the central part not bonded together, thus providing a high degree of flexibility. In order to have a high degree of flexibility, a thin, suitable coating, such as polyimide, can be applied to the wiring layer instead of a thicker laminate overlay.
  2. Finished soft insulation substrates. This category is manufactured on a soft insulating substrate, the finished product is specified to be flexible. This type of multilayer FPC is made of soft insulating material, such as polyimide film, laminated into a multilayer board, which loses its inherent flexibility after lamination.

FPC characteristics

  • Short: short assembly working time. All lines are configured. Eliminates the need for redundant line connections
  • Small: smaller than PCB. It can effectively reduce the size of the product. Increase the convenience of portability
  • Lightweight: lighter than PCBs (hard boards). It can reduce the weight of the final product.
  • Thin: thinner than PCB. It can improve flexibility. Enhances three-dimensional assembly in limited space

FPC advantages and disadvantages

Advantages of flexible printed circuit boards (FPC).
Flexible printed circuit boards are printed circuits made from flexible insulating substrates and have many advantages that rigid printed circuit boards do not have.
  1. Can be freely bent, wound, and folded, can be arbitrarily arranged in accordance with the requirements of the spatial layout, and can be arbitrarily moved and telescoped in three-dimensional space, thus achieving the integration of component assembly and wire connection.
  2. The use of FPC can greatly reduce the size and weight of electronic products, applicable to electronic products' high-density, miniaturization, and high-reliability direction of development needs. Therefore, FPC is widely used in aerospace, military, mobile communications, laptop computers, computer peripherals, PDAs, digital cameras, and other fields or products.
  3. FPC also has the advantages of good heat dissipation and solderability as well as easy to install even, low overall cost, etc. The combination of soft and rigid design also makes up for the slight shortfall in the component carrying capacity of flexible substrates to a certain extent.
 
Disadvantages of flexible circuit boards (FPC):
  1. High one-time initial cost: As flexible PCBs are designed and manufactured for special applications, so the start of the circuit design, wiring, and photographic negative is required for a higher cost. Unless there is a special need to apply flexible PCBs, usually a small number of applications, it is best not to use them.
  2. Flexible PCB changes and repair are more difficult: flexible PCB once made, to change the bottom of the map or the preparation of the photographic program must start, so it is not easy to change. Its surface is covered with a protective film, to remove before repairing, and to restore after repairing, which is more difficult work.
  3. Size is restricted: soft PCB in the case of not yet universal, usually manufactured by intermittent process, and therefore subject to the size of the production equipment, can not be made very long, or very wide.
  4. Improper operation is prone to damage: improper operation of the assembled company personnel is prone to cause damage to the soft circuit, its soldering and reworking require trained personnel to operate.

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